This invention relates generally to vertical optical cavity structures such as vertical cavity surface emitting lasers (VCSELs) and detectors (VCDETs) grown under lattice-relaxed conditions, and especially structures in which one of the Distributed Bragg Reflectors (DBRs) is grown under lattice-relaxed conditions.
Continued advances in long-distance, fiber optic communications depend on high-quality laser sources. Since optical fibers exhibit lowest attenuation and dispersion at the wavelengths of 1.3 xcexcm and 1.55 xcexcm suitable sources should emit at these relatively long wavelengths in single-mode operations.
Traditionally, long-wavelength distributed feedback (DFB) lasers are employed in fiber-optic communications systems for their single longitudinal and transverse mode characteristics. However, fabricating DFB lasers involves very complicated and low-yield processes. Furthermore, the DFB laser performance is very sensitive to the surrounding temperature change. Thus complicated electronics are needed in the transmitter to control the operating environment. These disadvantages render the DFB laser a very expensive light source and severely limit its application in the fiber-optic communications field.
Vertical Cavity Surface Emitting Lasers (VCSELs) emitting in the 1.3 xcexcm and 1.55 xcexcm ranges have been visualized as promising candidates for replacing DFBs in telecommunications applications. Due to their extremely short cavity length (on the order of one lasing wavelength), VCSELs are intrinsically single longitudinal mode devices. This eliminates the need for complicated processing steps that are required for fabricating DFB lasers. Furthermore, VCSELs have the advantage of wafer-scale fabrication and testing due to their surface-normal topology.
Unfortunately, VCSELs suffer material limitations that are negligible in the case of short-wavelength VCSELs but drastically affect the performance of long-wavelength VCSELs. The small available refractive index difference xcex94n between reflective layers of the Distributed Bragg Reflectors (DBRs) requires that a large number of layers with high composition and thickness precision be used to achieve sufficient reflectivity. Also, the small xcex94n results in high diffraction losses. Furthermore, high free-carrier absorption loss limits the maximum achievable reflectivity and the high non-radiative recombination rate increases the electrical current for reaching the lasing threshold.
These problems have restricted prior art fabrication efforts to non-wafer-scale, complicated and low-yield processes such as wafer fusion described by D.I. Babic et al., xe2x80x9cRoom-Temperature Continuous-Wave Operation of 1.54 xcexcm Vertical-Cavity-Lasersxe2x80x9d, IEEE Photonics Technology Letters, Vol. 7, No. 11, 1995, pp. 1225-1227 and Y. Ohiso et al., xe2x80x9c1.55 xcexcm Vertical-Cavity Surface-Emitting Lasers with Wafer-Fused InGaAsP/InP-GaAs/AlAs DBRsxe2x80x9d, Electronics Letters, Vol. 32, No. 16, 1996, pp. 1483-1484. Alternatively, long-wavelength VCSELs have also been manufactured by dielectric evaporation as described by S. Uchiyama et al., xe2x80x9cLow Threshold Room Temperature Continuous Wave Operation of 1.3 xcexcm GaInAsP/InP Strained Layer Multiquantum Well Surface Emitting Laserxe2x80x9d, Electronics Letters, Vol. 32, No. 11, 1996, pp. 1011-13; M. A. Fisher et al., xe2x80x9cPulsed Electrical Operation of 1.5 xcexcm Vertical-Cavity-Surface-Emitting Lasersxe2x80x9d, IEEE Photonics Technology Letters, Vol. 7, No. 6, 1995, pp. 608-610 and T. Tadokoro et al., xe2x80x9cRoom Temperature Pulsed Operation of 1.5 xcexcm GaInAsP/InP Vertical-Cavity Surface-Emitting Lasersxe2x80x9d, IEEE Photonics Technology Letters, Vol. 4, No. 5, 1992, pp. 409-411. Unfortunately, these methods do not allow one to efficiently grow long-wavelength VCSELs.
The prior art also addresses the problems associated with free-carrier absorption and non-radiative recombination which affect the threshold current. For example, in U.S. Pat. No. 5,034,958 Kwon et al. states that current confinement in VCSELs is necessary to improve current efficiency. Kwon also teaches that a high xcex94n material should be used for top and bottom DBRs. In U.S. Pat. No. 5,493,577 Choquette et al. further expand on the current confinement issue and teaches oxidation of the material layers of the VCSEL for this purpose. Still more information on this issue is found in U.S. Pat. No. 5,719,891 to Jewell.
Unfortunately, none of the prior art structures combines improved electrical properties-with high DBR reflectivity. Moreover, the fabrication processes are difficult and preclude one-step methods.
Some recent attempts focus on reducing the number of DBR layers while preserving high reflectivity by growing the DBRs on a suitable substrate such as InP. For example, O. Blum et al. teach the growth of AlAsSb/GaAsSb and AlAsSb/AlGaAsSb DBRs on InP in xe2x80x9cElectrical and Optical Characteristics of AlAsSb/GaAsSb Distributed Bragg Reflectors for Surface Emitting Lasersxe2x80x9d, Applied Physics Letters, Vol. 67, 27 November 1995, pp. 3233-35 and in xe2x80x9cDigital Alloy AlAsSb/AlGaAsSb Distributed Bragg Reflectors Lattice Matched to InP for 1.3-1.55 xcexcm Wavelength Rangexe2x80x9d, Electronics Letters, Vol. 31, No. 15, 1995, pp. 1247-8. Additional background information is also presented by T. Anan et al., xe2x80x9cImproved Reflectivity of AlPSb/GaPSb Bragg Reflector for 1.55 xcexcm Wavelengthxe2x80x9d, Electronics Letters, Vol. 30, No. 25, 1994, pp. 2138-9; B. Lambert et al., xe2x80x9cHigh Reflectivity 1.55 xcexcm (Al)GaAsSb/AlAsSb Bragg Reflector Lattice Matched on InP Substratesxe2x80x9d, Applied Physics Letters, Vol. 66, No. 4, 1995, pp. 442-3 and L. Goldstein et al., xe2x80x9cMetamorphic GaAs/AlAs Bragg Mirrors Deposited on InP for 1.3/1.55 xcexcm Vertical Cavity Lasersxe2x80x9d, LEOS Summer Topical Meetings, pp. 49-50, Montreal, Quebec, Canada, 1997.
Thus, although preferable characteristics required of a 1.3/1.55 xcexcm VCSEL for fiber-optic communications have been identified, there are no prior art techniques for combining them together in one, easy-to-fabricate device.
It is therefore a primary object of the present invention to provide a vertical cavity structure which combines the characteristics required for applications in the field of fiber-optic communications. Specifically, the device of the invention should be designed for efficient operation in the range from 1.3 xcexcm to 1.55 xcexcm. Moreover, the structure can be an active laser, i.e., a Vertical Cavity Surface Emitting Laser (VCSEL), or a Vertical Cavity Detector (VCDET).
It is another object of the invention to ensure that the structure is easy-to-fabricate, and in particular admits of being grown in one processing step.
Yet another object of the invention is to provide a method for growing vertical cavity structures exhibiting these advantageous characteristics.
Further objects and advantages will become apparent upon reading the specification.
These objects and advantages are attained by a vertical optical cavity which has a lattice-matched portion and a lattice-relaxed portion. The lattice-matched portion is grown to match a lattice of a lattice defining material, most preferably InP. This portion can include a bottom Distributed Bragg Reflector (DBR) and an active region grown on top of the bottom DBR. The lattice-relaxed portion has a predetermined lattice mismatch factor and includes a top DBR which is grown on top of the active region.
When the vertical optical cavity is to operate as a Vertical Cavity Surface Emitting Laser (VCSEL) the active region consists bulk active media, or a Quantum-Well region with at least one Quantum Well. Any of the commonly known types of Quantum Wells can be incorporated into the vertical cavity structure of the invention. For example, the Quantum Well can be a compressive strained Quantum Well, a tensile strained Quantum Well or an unstrained Quantum Well. Similarly, the Quantum Well barriers can be unstrained or strained. Alternatively, if the vertical optical cavity is to operate as a Vertical Cavity Detector (VCDET) the active region contains at least one filter layer.
The lattice mismatch factor in the lattice-relaxed portion can be as large as 20%. The bottom DBR is made of a material selected from among InAlGaAs, InGaAsP, AlGaAsSb. Meanwhile, the top DBR can be made of a material selected from among AlGaAs, InGaP and InGaAsP.
In a preferred embodiment the vertical optical cavity has an intermediate layer adjacent the active layer such as a current-confining layer. The intermediate layer can be lattice-matched to InP. The top, DBR can be partially oxidized to achieve higher reflectivity.
In another embodiment the vertical cavity has a tunable air gap adjacent the active layer. In this case the top portion can be suspended on a cantilever structure and thus the air gap permits one to tune the resonant wavelength of the vertical cavity.
The method of the invention allows one to grow a vertical optical cavity by an epitaxial growth method, such as Molecular Beam Epitaxy (MBE), in one step. In some cases the epitaxy technique for growing the lattice-relaxed portion of the cavity can differ from the technique used for growing the lattice-matched portion. Current confinement is achieved by controlled oxidation or ion implantation of appropriate layers of the cavity.
A detailed explanation of the invention is contained in the detailed specification with reference to the appended drawing figures.